The telecommunications industry is in the process of developing a new generation of flexible and affordable communications that includes high-speed access while also supporting broadband services. Many features of the third generation mobile telecommunications system have already been established, but many other features have yet to be perfected.
One of the systems within the third generation of mobile communications is the Universal Mobile Telecommunications System (UMTS) which delivers voice, data, multimedia, and wideband information to stationary as well as mobile customers. UMTS is designed to accommodate increased system capacity and data capability. Efficient use of the electromagnetic spectrum is vital in UMTS. It is known that spectrum efficiency can be attained using frequency division duplex (FDD) or using time division duplex (TDD) schemes. Space division duplex (SDD) is a third duplex transmission method used for wireless telecommunications.
As can be seen in FIG. 1, the UMTS architecture consists of user equipment 102 (UE), the UMTS Terrestrial Radio Access Network 104 (UTRAN), and the Core Network 126 (CN). The air interface between the UTRAN and the UE is called Uu, and the interface between the UTRAN and the Core Network is called Iu.
The UTRAN consists of a set of Radio Network Subsystems 128 (RNS), each of which has geographic coverage of a number of cells 110 (C), as can be seen in FIG. 1. The interface between the subsystems is called lur.
Each Radio Network Subsystem 128 (RNS) includes a Radio Network Controller 112 (RNC) and at least one Node B 114, each Node B having geographic coverage of at least one cell 110. As can be seen from FIG. 1, the interface between an RNC 112 and a Node B 114 is called Iub, and the lub is hard-wired rather than being an air interface. For any Node B 114 there is only one RNC 112. A Node B 114 is responsible for radio transmission and reception to and from the UE 102 (Node B antennas can typically be seen atop towers or preferably at less visible locations). The RNC 112 has overall control of the logical resources of each Node B 114 within the RNS 128, and the RNC 112 is also responsible for handover decisions which entail switching a call from one cell to another or between radio channels in the same cell.
LTE, or Long Term Evolution (also known as 3.9G), refers to research and development involving the Third Generation Partnership Project (3GPP) aimed at identifying technologies and capabilities that can improve systems such as the UMTS. The present invention is related to LTE work that is taking place in 3GPP.
Generally speaking, a prefix of the letter “E” in upper or lower case may signify LTE, although this rule may have exceptions. The E-UTRAN consists of eNBs (E-UTRAN Node B), providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs interface to the access gateway (aGW) via the S1, and are inter-connected via the X2.
An example of the E-UTRAN architecture is illustrated in FIG. 2. This example of E-UTRAN consists of eNBs, providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the SI interface to the EPC (evolved packet core) more specifically to the MME (mobility management entity) and the UPE (user plane entity). The SI interface supports a many-to-many relation between MMEs/UPEs and eNBs. The S1 interface supports a functional split between the MME and the UPE. The MMU/UPE in the example of FIG. 2 is one option for the access gateway (aGW).
In the example of FIG. 2, there exists an X2 interface between the eNBs that need to communicate with each other. For exceptional cases (e.g. inter-PLMN handover), LTE_ACTIVE inter-eNB mobility is supported by means of MME/UPE relocation via the S1 interface.
The eNB may host functions such as radio resource management (radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to UEs in both uplink and downlink), selection of a mobility management entity (MME) at UE attachment, routing of user plane data towards the user plane entity (UPE), scheduling and transmission of paging messages (originated from the MME), scheduling and transmission of broadcast information (originated from the MME or O&M), and measurement reporting configuration for mobility and scheduling. The MME/IUPE may host functions such as the following: distribution of paging messages to the eNBs, security control, IP header compression and encryption of user data streams; termination of U-plane packets for paging reasons; switching of U-plane for support of UE mobility, idle state mobility control, SAE bearer control, and ciphering and integrity protection of NAS signaling.
The present invention is related to intercell interference in LTE, although the solution of the present invention may also be applicable to present and future systems other than LTE. One possible access technique in LTE for the downlink connection is orthogonal frequency division multiplexing (OFDM), applying different system bandwidths from 1.25 MHz to 20 MHz. OFDM splits the datastream into multiple radio frequency (RF) channels, each of which is sent over a subcarrier frequency. The signal-to-noise ratio of each of those very precisely defined frequencies is carefully monitored to ensure maximum performance.
According to this OFDM approach, the frame structure would divide a frame of 10 ms into a number of sub-frames, each having a duration of 0.5 ms. Each of these sub-frames will consist of a number of OFDM symbols, which will be either 6 or 7. The 7 OFDM symbols per sub-frame will carry unicast transmissions, while the 6OFDM symbols subframe will carry multicast transmissions. However, it should be borne in mind that the multicast/unicast definiton could potentially be altered in the future (e.g. 6 OFDM symbols could be used for unicast transmission).
The present invention is concerned primarily with unicast transmission, but in principle multicast transmission could be addressed in a similar way, especially in the unlikely event that hybrid automatic repeat request (H-ARQ) for multicast is implemented. The current working assumption is that the first OFDM symbol within a sub-frame will hold the essential information to ensure proper operation of a cell (that is, the pilot symbols for proper channel estimation as well as allocation information-which describes the allocation of the physical resources to the different users within the cell). This first OFDM symbol will carry common information as well as shared control information, and in order to ensure that this information is provided to the entire coverage area of the cell, this OFDM symbol has to be transmitted at relatively high power, and will have to be transmitted for every sub-frame. One of several ways to arrange this system is for a transmission time interval (TTI) to consist of a sub-frame pair (1 ms duration), and for the control channel information to be located within the first 3 OFDM symbols (or even less), although many other arrangements are possible (as will be understood by a person skilled in the art).
It is possible for the physical layer (providing access to the radio channel) to provide hybrid automatic repeat request (H-ARQ) to recover from reception errors. This is a well-known technique in modern communications, and will compensate for errors in the radio channel as well as measurement errors in the feed-back loop. Furthermore, it is currently assumed that the Node B's of the network are operating in unsynchronized mode to simplify the network. Hybrid automatic repeat request (H-ARQ) retransmissions should preferably experience a lower amount of interference conditions than is currently possible according to prior art technology.
A problem with the prior art is that the “first OFDM symbol within a sub-frame” from other cells will typically be offset in time relative to the current reception from the serving cell. This means that the interference for a single user or cell will typically be bursty, but periodical and with a constant time offset. Furthermore, when there is more than a single interfering cell, it might be difficult to circumvent the problem in a simple way.
This problem will be most clear in lightly loaded cells, but can also occur whenever (or if) slow power control is applied in the downlink. One case of interest is the case of Voice Over Internet Protocol (VoIP), where many users may be multiplexed. In this case the control symbol—or the first OFDM symbol—needs to be “filled” out to do the addressing, but the required data amount (e.g. maximally 10-15VoIP packets) will not fill out the other data symbols. Thus, we have some multiplexing freedom available, but the prior art has not exploited that freedom.
When considering the control channel overhead for the situation with many low-data rate users, it is important to keep in mind that the control channel (and the common/broadcast channels) needs to be transmitted all the time. Also, for instance, the synchronization channel and the pilot channel need to be transmitted using quite high power levels in order to assure proper detection by all mobiles within the cell.